RHS Level 2 Certificate

Year 2 Week 20 – Fungal, bacterial and viral plant diseases and physiological disorders.

Learning Objectives

4. Know the problems posed by diseases in horticulture and ways in which these can be minimised.

4.1 Define ‘plant physiological disorder’ and describe TWO such disorders4.2 Define ‘plant disease’. 4.3 Describe the damage caused by grey mould, powdery mildew,

damping off, honey fungus, rose black spot, potato blight and clubroot. 4.4 Describe TWO different methods of reducing the effects of EACH of

the diseases stated in 4.2; methods should be selected from more than one of the control options (chemical, physical, cultural or biological) available.

4.5 Describe the damage caused by fire blight and describe TWO methods used to limit the spread of the disease.

4.6 Describe the damage caused by TWO named plant viruses. 4.7 Describe TWO methods by which viruses can be spread. 4.8 Describe TWO methods of avoiding the spread of plant viruses.

Physiological Disorders

Plant problems caused not by pests or disease but by some problem in the environment.

For example: frost causes browning of foliage; strong winds, especially in winter when the ground is frozen, can cause die back in conifers.

Fasciation: distorted growth due to damage at the growing tips of stems, flowers and fruit.

Nutrient deficiencies – e.g. interveinal chlorosis in Azalea caused by iron deficiency in high pH soil.

Plant Disease

Damage caused to plants by some disease causing agent – bacteria, viruses or fungi. Collectively disease causing agents are known as pathogens.

Plants do not have an immune system, unlike animals. If an individual plant survives a disease it will not be more resistant to another attack in the future.

Some cultivars however are genetically resistant to disease – they are less likely to get it or less affected than others in the same species.

Bacterial Disease

Bacterial diseases colonise plant tissues, giving off toxic compounds that kill plant cells and feeding on the remains.

No chemical controls available – growing resistant varieties and cultural controls are the only solutions.

Not all bacteria are harmful – without nitrogen fixing and nitrifying bacteria the Earth would be a very different place with far fewer plants.

Bacterial Disease - Fireblight Bacteria enter through the

nectaries. Flowers and foliage die and

blacken but often do not fall – scorched appearance.

Affects plants in the family Rosacea – Apples, Pears, Roses, Rowan, Hawthorn

Control – prune back to sound wood and burn infected material.

Viral Diseases Viruses are the ultimate parasites – using living

cells by taking over their processes to replicate viral DNA.

Present in every part of an infected plant, even if not showing symptoms.

Spread by vegetative propagation from infected plants.

Also spread in sap by aphids and transferred between plants on tools and hands.

No chemical controls available.

Tobacco Mosaic Virus (TMV)

Leaf mottling, poor growth, small fruit.

Spread by contact – on hands or tools.

Also spread in seed. Controls – resistant

varieties; clean hands; no smoking; clean tools; certified virus free seed.

Carrot motley dwarf disease

A combination of two viruses spread together.

Causes yellowing or reddish colour on leaves and stunting.

Spread by willow-carrot aphid which flies in May –so early carrots and late ones least at risk

Control of aphid controls the virus.

Fungal diseases

Plants under stress are most susceptible to fungal infection – so growing healthy plants in the right place is a good control.

Many problem fungi are normally resident in dead and decaying plant material – moving to living plant tissue through damage to stems (e.g. Clematis wilt) or via dying leaves or flower spikes.

Some garden fungi are beneficial – mycorrhizal fungi form beneficial symbiosis with many plant roots

Grey Mould Botrytis cinerea

Wind spread spores colonise plant tissues in humid conditions producing fluffy white growth

Black fruiting bodies form which will split to release spores

Controls: no chemical; control humidity; cultural hygiene

Powdery Mildew

White or grey dusty growth on leaves and shoots; weaken the plant

Controls: cultural hygiene to remove spores; avoid water stress; chemicals include myclobutanil (systemic) and sulphur dust (not for sulphur shy plants like gooseberries)

Damping Off disease. Pythium sp., Phytopthera sp and

Rhizoctonia) Fungal disease of

seedlings; causes them to rot at the base and die.

Encouraged by thick planting and cold wet soil.

Controls: Control humidity by not overwatering and sow thinly; use copper oxychlorate as a drench.

Honey Fungus Severe fungal infection that attacks both

dead and living wood. Spread by tough black ‘bootlaces’ through

the soil and then penetrates roots and grows into the tree.

Visible signs are yellow or pale brown mushrooms and sudden wilting or death of plant

Controls – remove and burn all parts of the affected plant; use soil barriers to prevent spread

Learning outcomes4. Know the problems posed by diseases in horticulture and ways in

which these can be minimised. 4.1 Define ‘plant physiological disorder’ and describe TWO such

disorders4.2 Define ‘plant disease’. 4.3 Describe the damage caused by grey mould, powdery mildew,

damping off, honey fungus, rose black spot, potato blight and clubroot.

4.4 Describe TWO different methods of reducing the effects of EACH of the diseases stated in 4.2; methods should be selected from more than one of the control options (chemical, physical, cultural or biological) available.

4.5 Describe the damage caused by fire blight and describe TWO methods used to limit the spread of the disease.

4.6 Describe the damage caused by TWO named plant viruses. 4.7 Describe TWO methods by which viruses can be spread. 4.8 Describe TWO methods of avoiding the spread of plant viruses.